In a continuous cooking process fibrous material, such as wood chips, saw dust or the like material, is fed into the upper part of an upright digester vessel, in which delignification is carried out at an increased temperature and pressure (8-10 bar).
Pulp is cooked normally at a temperature of about 170.degree. C. The fibrous material and the cooking liquor are normally introduced into the digester at a temperature of less than 100.degree. C. Steam is usually used for heating the fibrous material to the cooking temperature of 170.degree. C. Heating may be carried out stage by stage in such a way that the fibrous material is first heated by low pressure steam to about 120.degree. C., and later in the second stage by high pressure steam to about 170.degree. C.
The products of the cooking are hot discharged waste liquor, which has a temperature of about 170.degree. C. and hot pulp. Several methods are used in the industry to recover the heat content of waste liquor. A common way of utilizing the heat content of the discharged waste liquor is to let the waste liquor evaporate rapidly by reducing the pressure and to utilize the vapor generated thereby for heating wood chips or for evaporation as is shown in FIG. 3 of U.S. Pat. No. 3,286,763. Utilization of the energy content of waste liquor by this method is not optimal. The reduction of the temperature required by the rapid flash evaporation becomes on the one hand unnecessarily sharp and on the other hand the temperature of the vapor generated thereby is unnecessarily low. Swedish patent application 8503282-9 discloses the treatment of cooking liquor from a digester during the cooking step. The cooking liquor is withdrawn, evaporated with steam and the evaporated vapor and the cooking liquor are returned to the digester.